1. Field of the Invention
The present invention relates to a photoelectric displacement detector and, more specifically, to a photoelectric displacement detector capable of producing a stable displacement detection signal by correcting variations of a DC level of the same for thereby raising interpolations accuracy and improving a responsive scanning speed.
2. Description of the Related Art
A photoelectric type displacement detector is known in a field where amounts of feeding of tools in machine tools and other like quantities are measured. In such a photoelectric displacement detector, a main scale, on which a periodic main grating is formed, is fixed on one of opposite members, and a detector is fixed to the other member, the detector including an optically transparent index scale, on which a periodic sub grating is formed correspondingly to the main grating, an illuminating optical system composed of a light source, and an optical detector for photoelectrically converting a light from the illuminating optical system which has been modulated by the main grating and the sub grating, whereby a periodically varying detection signal is generated in response to a relative displacement between both members.
Referring to FIG. 21, a conventional reflecting photoelectric displacement detector is exemplarily illustrated. The detector includes a light emitting diode (LED) 10 as a light source, a collimator lens 12 for collimating a light emitted from the LED 10 into a parallel illuminating light, a main scale 14, on which a periodic main grating 16 is formed, an optically transparent index scale 18 disposed relatively movably with respect to the main scale 14 and having a periodic sub grating 20 formed thereon corresponding to the main grating 16, and an optical detector element 22 for displacement detection which photoelectrically converts a reflected light R from the collimator lens 12, the light R being reflected by the main grating 16 of the main scale 14 and transmitted by the sub grating 20 of the index scale 18. The detector thereby generates a periodic detection signal responsively to a relative displacement between the main scale 14 and the index scale 18.
In such a photoelectric displacement detector, the sub gratings 20 and the optical detector elements 22, amount for example is four in total, respectively, as illustrated in FIG. 22, two on the index scale 18 in a direction parallel to graduations on the grating (vertical direction in the figure) and two on the same index scale longitudinally of the main scale (horizontal direction in the figure). Assume, for example, the phase of the sub grating 20a to be a reference "0", that of the sub grating 20b to be -90.degree., that of 20c to be +180.degree., and 20d to be -90.degree., the optical detector elements 22a, 22c, and 22b, 22d disposed diagonally on the detector with respect to the longitudinal direction of the main scale generate difference displacement detection signal (a-c), (b-d), two phase detection signals as shown in FIG. 23, among displacement detection signals a to d detected by the four optical detector elements 22a to 22d disposed correspondingly to the sub gratings 20a to 20d. Here, in FIG. 22, designated at 24, 26 are differential amplifiers.
The conventional photoelectric displacement detector assures as described above two detection signals different in their phases 90.degree. from each other for discrimination of the direction of the displacement and for a precision measurement by electrical interpolation. In this situation, by the use of the aforementioned differential detection system, variations of a DC level of the displacement detection signals and phase variations due to a change in parallelism between the main scale 14 and the index scale 18 can be corrected.
However, the main scale 14 is, if in a continuous length of 300 mm or more for example, constructed in general by first positioning it on a stepper, and then transcribing an original scale of a short length on it, and exposing it to a light length by length. Accordingly, the thickness (depth) of chrome of grating graduations is varied as it goes longitudinally, which results in variations of reflectivity and transmittance of the scale and slight variations of the line width of the graduations, making it very difficult to form the scale uniformly. In the main grating of the main scale of 8 micrometers pitch for example, variations of a detection signal sometimes amount to about 10% because of the line width being variable by about 0.2 micrometer or about 2.5% together with other undesirable factors.
Such a conventional photoelectric displacement detector has some problems as follows. Existence of such longitudinal variations causes a change in a DC level of a detected output. Further, even with the aforementioned differential technique, correction of variations of the DC level is unsatisfactory if there is any difference between variations of DC levels of difference signals of opposite phases. Particularly, in a reflecting displacement detector wherein a light source is provided at the center of sub gratings, an interval is increased between two optical detectors of different two phases (corresponding to the sub gratings 20a and 20c, and 20b and 20d), followed by a severely large relative difference between variations of DC levels.